Until now so-called fully electric or hybrid drive systems were used for ships, the drive systems of which have to be designed and optimized for a number of speeds or speed ranges due to their travel profile. Such drive systems generally include at least two drive motors operated independently of one another. They generally drive the ship propulsion system by way of a power transmission unit.
The power transmission unit can comprise the following components for example: a shaft system with associated bearings and a thrust bearing, a coupling between drive motor and shaft system as well as a step-down gear between drive motor and shaft system. The ship propulsion system can in particular comprise a fixed pitch propeller or a variable pitch propeller system.
In the known prior art the electric traction motors used in the fully electric or hybrid drive systems are generally supplied from a drive power system at constant voltage and frequency. To regulate the rotational speed of the electric traction motors, they are generally supplied by way of a converter which converts the fixed voltage and frequency to a variable voltage with variable frequency, which is in each instance between almost zero and a maximum voltage or maximum frequency. The logical consequence here is that all the drive power must be made available by corresponding converters and transformers. However converters and transformers represent a significant weight and volume with the consequence that for example weight-critical ship types, for example lightweight ships, could hitherto not be fitted with such drive systems.
Alternatively a ship unit system can be provided that comprises at least one generator in a drive power system and drives the ship by way of at least one electric shaft, the electric shaft being connected to a ship propulsion system and comprising at least one traction motor. The drive power system here refers to the electric power system which supplies the drive power for the ship. Two or more generators, also called drive power system generators and driven for example by diesel sets or gas turbines, are preferably used for this purpose. The traction motor comprises a jet and a propeller for example. The electric shaft refers to a fixed electric coupling between generators and drive motors, which is such that a rotational movement of the generators brings about a corresponding rotational movement of the electric drive motors. The function of a mechanical shaft is reproduced with such an electric shaft. The generators of the drive power system can be connected electrically and are matched to the traction motor(s) in respect of pole number and voltage.
The generators can be connected both to one another and also to the traction motors, provided that the respective voltages and frequencies are synchronized. A transformer with at least one converter is also connected to the drive power system, being switched both to the on board power system and to a propeller motor, the on board power system representing the residual electric power system of the ship, which supplies for example the electric power for lighting, navigation and control devices and the like. As the on board power system generally also has its own energy generation system, preferably for emergency and port operation, and is supplied for example using multifuel combustion sets and connected generators, it is also possible additionally to supply the traction motor from the on board power system up to a certain rotational speed of the traction motor. An on board power system inverter or a further power inverter can be used for this purpose, converting the on board power system voltage, an alternating voltage at constant frequency and constant amplitude, to an alternating voltage of variable frequency and variable amplitude for the operation of the traction motor. This power inverter is also referred to as a start-up converter and can in principle also be operated in the reverse manner so that the variable alternating voltage of the drive power system is converted to the on board power system voltage.
It is necessary to supply the traction motor from the on board power system, as the electric shaft cannot be operated below a certain frequency due to the idle speed of the internal combustion engines. As a result the propulsion system cannot be operated smoothly in respect of its rotational speed from the stationary state to the idle speed of the internal combustion engine using a drive system supplied exclusively by an electric shaft. This means that at idle speed when the electric shaft is activated the torque acts on the ship propulsion unit and therefore the ship's speed or the maneuverability of the ship is unsatisfactory when traveling slowly, for example during port operation and when maneuvering. It is also desirable for the torque to act constantly on the ship's propeller, which affects the switching point at idle speed with an increasing or decreasing rotational speed.
Such an alternative ship unit system as cited above is known from WO2011092330A2, wherein there is provision for a number of electric drive shafts and for drive motors to be supplied by way of the electric shafts or by way of an on board power system.
In one operating state, a drive propeller of the ship can be driven by way of at least one electric shaft, which comprises an internal combustion engine and a generator in each instance, the internal combustion engine and therefore also the drive system for the drive propeller having a minimum rotational speed in this operating state. In a different operating state, in which the propeller rotational speed is lower than this minimum rotational speed, the drive propeller can be supplied by way of the on board power system, in particular with an alternating voltage at low frequency. This allows a fixed pitch propeller to be used even when low ship speeds are desired.